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5. Remove Element

easyAsked at Udemy

Strip occurrences of a value in place — Udemy uses this to test pointer mechanics before harder catalog-cleanup problems.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an array nums and an integer val, remove all occurrences of val in place and return the new length k. The first k elements of nums may be in any order.

Constraints

  • 0 <= nums.length <= 100
  • 0 <= nums[i], val <= 50

Examples

Example 1

Input
nums=[3,2,2,3], val=3
Output
2, nums=[2,2,_,_]

Example 2

Input
nums=[0,1,2,2,3,0,4,2], val=2
Output
5, nums=[0,1,3,0,4,_,_,_]

Approaches

1. Filter and copy

Filter non-val into a new array, then copy back.

Time
O(n)
Space
O(n)
const keep = nums.filter(x => x !== val);
for (let i = 0; i < keep.length; i++) nums[i] = keep[i];
return keep.length;

Tradeoff:

2. Two-pointer in place

Use a write pointer; advance read pointer and copy forward only when nums[i] !== val.

Time
O(n)
Space
O(1)
function removeElement(nums, val) {
  let k = 0;
  for (let i = 0; i < nums.length; i++) {
    if (nums[i] !== val) nums[k++] = nums[i];
  }
  return k;
}

Tradeoff:

Udemy-specific tips

Udemy may reframe this as scrubbing flagged-review entries from a course rating list — show you keep the pattern simple and don't allocate copies.

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Output

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